Tightening devices of this type usually comprise an epicyclic train coupled to a drive assembly and having a planet gear support frame and an internal gear. An inner socket and an outer socket are connected to the frame and the gear, resectively, such that a nut can be tightly screwed on a bolt by the reaction of rotation acting on the two sockets.
However, if the bolt or the nut is provided with a friction member in between for preventing loosening or if there is deformation or a flaw on the bolt which prevents smooth rotation of the nut and when the tightening device is brought into operation before the nut comes into intimate contact with the workpiece P as seen in FIG. 6, it is likely that the bolt and the nut rotate together, permitting the outer socket 71 and the inner socket 8 to rotate at a high speed in a direction opposite to the nut tightening direction without slowing down to surprise the operator. This is attributable to the absence of the reaction of rotation that otherwise would act between the inner socket and the outer socket, in which case the internal gear, sun gear and planet gears fail to rotate relative to one another, allowing the epicyclic train to rotate in its entirety at the speed of rotation of the sun gear without operating as designed.